Many recent technological advances are made possible by a myriad of electronic devices that function in countless operational devices to facilitate fast and efficient data processing and communication. Modern commerce, and all the associated production, distribution, and communication, could not be achieved without these electronic devices. The circuit board is central to each of these electronic devices.
The increased use of circuit boards in modern electronic devices makes it desirable to produce them with less cost, and ways to produce boards more efficiently are sought by the industry. Yet, as electronic devices are challenged with greater and new tasks, improvements in efficiency and speed of performance are also sought. This places increasing demands on the circuit boards in the devices. The performance and speed of circuits built on the board are directly affected by the dielectric constant of the material from which circuit boards are formed. A material having a lower dielectric constant permits better performance and higher speeds. Thus, to meet the demand for increased speed and higher performance, substrate materials with lower dielectric constants are sought.
Circuit Boards
The National Electronic Manufacturers Association (NEMA) classifies circuit boards on the basis of their composition. Such boards are referred to as FR-1, FR-2, CEM-1 etc; see table 1. A conventional and frequently used printed circuit board, an "FR-4" board, is a laminated composite made of fiberglass fabric impregnated with an epoxy resin.
Circuit boards such as FR-4 boards are produced by impregnating fiberglass fabric with a liquid thermosetting epoxy resin. The impregnated fabric is heated to partially cure the resin and to form a dry, flexible sheet in which the resin is in an intermediate cure state, termed the "B"stage. This dry sheet is termed a "pre-preg" sheet. In further manufacturing steps, sheets of pre-preg are stacked together to a desired thickness and are subjected to heat and pressure that fully cures the resin. This forms a laminated composite in which the resin is said to be in the "C"-stage.
Other conventional methods of producing pre-pregs involve treating paper made from cotton linters and/or bleached kraft fibers with a saturant that is a solution of epoxy resin, catalyst, flame retardant and other ingredients. The saturated paper is then dried by beating to an intermediate temperature to form a B-stage pre-preg. About 99% of the volatile components of the saturated paper are removed by this heating. Laminates are made by stacking three or more of the B-stage pre-pregs together and flanking them on both sides by epoxy-saturated glass cloth, which in turn are flanked by copper foil. A double-clad laminate is formed with copper foil on both sides and a single-clad laminate has copper foil on a single side. The stacked laminate is then pressed between platens and heated to a high temperature for a certain time. When pressed and heated to high temperatures, the resin becomes fluid, and flows to uniformly penetrate the resulting board. Above a certain temperature, crosslinking of the resin is catalytically initiated, and a hard and tough composite structure C-stage laminate is formed. C-stage laminates are usually cooled while under pressure in order to preserve their flatness. The lamination process that converts the B-stage epoxy resin of the pre-preg into C-stage resin also achieves insulation of the circuit layers.
TABLE 1 ______________________________________ Types of Conventional Circuit Boards GRADE COMPOSITION APPLICATION ______________________________________ FR-1 Phenolic resin/Kraft paper Automotive Consumer electronics FR-2 Phenolic resin/Kraft paper Automotive Video game controls Consumer electronics FR-3 Epoxy resin/Kraft paper Telephone sets FR-4 Epoxy resin/Glass fabric Computer applications Telecommunications Military FR-5 Modified Epoxy resin/Glass fabric Military applications CEM-1 Epoxy resin/Glass fabric surface Consumer electronics Epoxy resin/Cotton and/or bleached kraft paper core CEM-3 Epoxy resin/Glass fabric surface Computer peripherals Epoxy resin/Glass paper core Keyboards ______________________________________
During production of the laminate for printed circuit boards, sheets of pre-preg are usually stacked and bonded to one or two sheets of copper foil so that the final laminated composite consists of dielectric material clad on one or both sides with copper foil. This laminated composite single-clad or double-clad material is fabricated into single- or double-sided printed circuit boards.
Pre-pregs are also used in the production of multilayer printed circuit boards that are used where very high circuit densities are needed. To make multilayer printed circuit boards, "innerlayers" of FR-4 copper clad laminate are fabricated into single- or double-clad circuit boards that are interleaved with one or more sheets of B-stage pre-preg. These multilayer structures are then laminated together under heat and pressure to form C-stage resin that insulates and bonds the innerlayers together.
The electronic systems of modern equipment typically have high signal speeds and operating frequencies. Electronic circuit boards made of materials with low dielectric constants have higher resistivity and decreased capacitive coupling. Higher resistivity and decreased capacitive coupling permit the speed of electronic signal transmission to be increased. Data can be processed at greater speeds. Thus, by using a board with a lower dielectric constant and increased resistivity (compared with alternative materials) the system may be designed with a higher speed of processing electric signals and reduced electric power losses.
Dielectric Constants of Circuit Boards
The dielectric constant (or E.sub.r) of a material is a physical characteristic of the material. Printed circuit boards are commonly made of substrates such as paper, nonwoven glass mats and woven glass cloth saturated with polymeric resins. Less common substrates are aramide fibers (woven and nonwoven) and polytetrafluoroethylene (PTFE) cloth.
Standard FR-4 laminate has a relatively high E.sub.r, approximately 4.2 at 1 megahertz. This a result of the high E.sub.r contribution of the fiberglass, about 6, averaged with the lower E.sub.r of the epoxy resin, about 3.4.
The polymeric resins used in printed circuit boards are commonly phenolic resins, epoxy resins and modifications of these, e.g. polyesters, cyanoesters, polyimides and PTFE. FR-1, FR-2, FR-3, CEM-1, CEM-3 and FR-4 circuit boards are made from these common materials. The E.sub.r of these printed circuit boards are typically in the range of 4 to 5.
Laminated composites comprised of fiberglass fabric impregnated with fluorocarbon resins are also used in the electronics industry. These laminated composites have an E.sub.r of approximately 2.5 at 1 megahertz. However, fluorocarbon resins suffer from the disadvantage that they are not thermosetting. Composites impregnated with fluorocarbon resins are difficult to fabricate into multilayer printed circuit boards. Pre-preg sheets prepared with fluorocarbon resins can be bonded only at temperatures at which innerlayers melt and lose their dimensional stability.
Other dielectric materials for special purposes are made that particular fibers that have a low E.sub.r. Polyaramide fibers together with epoxy resins yield composites with an E.sub.r of about 3.8, and in general aramide boards may have an E.sub.r in the range of 3 to 4. Quartz fibers have also been used in composites, to give materials with E.sub.r s similar to those made with polyaramide fibers.
Printed circuit boards made from materials such as PTFE may have an E.sub.r of less than 3. However, some of these "exotic" printed circuit boards are difficult to process and have other properties such as poor mechanical properties that limit their usefulness. For instance, non-woven aramide fibers have high water absorption, and although PTFE has a low dissipation factor, it also has a low mechanical strength and poor adhesive properties. Also, aramide fabrics and fibers, such as PTFE, are expensive and the cost of boards made from exotic materials is considerably higher than with the common substrates discussed above. It is therefore very desirable to develop alternate substrates for the manufacture of FR-1, FR-2, FR-3, CEM-1, CEM-3 and FR-4 boards that possess an E.sub.r in the range of 3-3.5. There is also a need for boards with low dissipation factors, which are easier to process than those presently available and which provide good mechanical and electrical properties.
Substrates with a low dielectric constant can be made in various ways. For example, polyester fibers (E.sub.r of less than 3) can be used instead of glass fibers (E.sub.r of about 6). A drawback of such materials is low resistance to heat and their tendency to melt and loose their desirable properties at higher temperatures. Also polyester fiber has a lower strength relative to glass fiber, which yields less desirable mechanical properties such as dimensional stability.
The dielectric constant of the material from which the board is formed has a direct effect on the performance and speed of circuits built on the board. The increased electrical performance required of circuit boards in modern equipment has prompted a search for ways to reduce the dielectric constant of the materials from which they are made. Air has the lowest E.sub.r of all materials used , but in order for it to be effective it must be stably incorporated within the circuit board. Incorporation of air into a circuit board has been achieved by encapsulating the air in glass in the form of very small, sturdy spheres (hollow glass microspheres). This approach has been used by a number of laminators who mixed hollow glass microspheres made by 3M Co. or the W. R. Grace Co., subsidiary, Emerson & Cuming, with the resin component of the pre-preg.
Chellis et al. (U.S. Pat. No. 5,126,192) teaches hollow glass microspheres added to a resin which is then formed into parts for use in electronics applications that have low E.sub.r and other desirable attributes, the so-called "champagne board." Chellis discloses that very small microspheres may confer a reduced buoyancy on a product relative to larger glass spheres, and that this advantage is gained especially when spheres of about 5 to 25 microns are used. Chellis also discloses the disadvantage that microspheres may be buoyant and require continuous agitation to keep them suspended. Additionally, Chellis discloses that low-shear mixing techniques must be used to avoid damaging the microspheres.
Okada et al. (U.S. Pat. No. 4,661,301) teaches adding glass microspheres to a resin from which flat plates are made by an extrusion process. Okada also discloses a circuit board produced by flanking the extruded flat sheet on one or both sides with a reinforcing resin-saturated fabric and laminating it with copper foil.
Polyclad Corp. recently advertised a low-dielectric-constant reinforced laminate system with hollow glass microspheres distributed throughout a resin matrix. (CircuiTree, pp 42-43, October 1996). The Polyclad material is stated to have a superior E.sub.r without the use of exotic, expensive resins and to yield both a cost reduction and an improvement in quality. Other similar uses of glass spheres in resins for printed circuit board applications are known (U.S. Pat. Nos. 5,308,909, and 5,098,781). All of these methods involve suspending the glass microspheres in the resin component of the board, i.e. the saturant. These methods require that the resin be continually mixed so as to avoid an uneven distribution of microspheres in the resin. Continual mixing tends, however, to damage microsphere fillers during processing.
In the prior art, paper is used in the production of CEM-1 and a variety of FR-boards. In the past, paper for making circuit boards was typically composed of cotton linter fibers, but such paper is now predominantly made from wood fibers. Composites made with such papers are economically attractive and yield boards with good physical properties such as warp, twist and punching. However, boards made from wood fibers have high dielectric constants and low heat dissipative capacities that limit their use to applications for less demanding conditions.